A mobile communication system having multiple cells continues communications by switching serving cells when a user equipment (UE) moves from a region in one cell to a region in another cell. This switching of serving cells is called a “handover.”
Generally, when the user equipment moves to an adjacent cell and a signal from the adjacent cell becomes stronger than a signal from the serving cell, a handover to the adjacent cell is performed.
For example, the user equipment performs a handover according to the procedures shown in FIG. 1.
In Step S1, the user equipment measures communication quality in the adjacent cell (signal power of the adjacent cell).
In Step S2, the user equipment checks if the communication quality in the adjacent cell (signal power of the adjacent cell) satisfies the following formula.Signal Power of Adjacent Cell+Offset>Signal Power of Serving Cell
Then, when the above formula is satisfied, the user equipment reports the result (Event A3) to a base station device (network).
Such a report to the base station device is made through a “measurement report.”
Note that the offset is a value provided to prevent a handover from the serving cell to the adjacent cell from frequently occurring at the cell boundary. The offset value may be either positive or negative. In order to prevent a handover from the serving cell to the adjacent cell from frequently occurring at the cell boundary, the offset value is generally set to negative in the above formula.
In Step S3, upon receipt of the measurement report about Event A3 described above, the base station device determines that the user equipment should perform a handover to the adjacent cell for which Event A3 described above has been reported, and executes the handover procedures.
In other words, the base station device sends a message instructing the user equipment UE to perform a handover, i.e., a handover command to the user equipment UE.
Here, Event A3 described above is an event about measurement of the adjacent cell having the same frequency as the serving cell.
Note that a long term evolution (LTE) system that succeeds a wideband code division multiple access (W-CDMA) system and a high speed downlink packet access (HSDPA) system uses a “reference signal received power (RSRP)” as one of the criteria (the signal powers from the adjacent cell and the serving cell in the above example) for determining whether or not to perform a handover.
Here, besides the RSRP, a “reference signal signal-to-interference ratio (RS-SIR),” an “E-UTRA carrier received signal strength indicator (RSSI),” a “reference signal received quality (RSRQ)” and the like may be used.
Incidentally, although a handover destination is the cell having the same frequency in the above example, the handover destination may be not only the cell having the same frequency in the same system but also a cell having a different frequency in the same system or a cell using a different radio access technology (RAT).
The cell using a different radio access technology generally has a frequency different from that of a handover source. Therefore, the frequency of the handover destination cell naturally differs from the frequency of the handover source cell.
FIG. 2 schematically shows how a handover is performed between cells having different frequencies. FIG. 2 shows an LTE mobile communication system including a mobile communication system using a first frequency f1 and a mobile communication system using a second frequency f2, and a W-CDMA mobile communication system using a third frequency f3 different from f1 and f2.
For example, in FIG. 2, the base station device can instruct the user equipment communicating with the mobile communication system using the first frequency f1 to perform measurement on cells of two layers, including a cell of the second frequency f2 and a cell of the third frequency f3.
Note that, in the following description, each of the frequencies, such as the first frequency f1, the second frequency f2 and the third frequency f3, is called a layer. Specifically, there are three layers in FIG. 2, which are first to third layers corresponding to the first, second and third frequencies f1, f2 and f3, respectively.
Generally, the user equipment includes only one radio signal processor and thus cannot simultaneously transmit and receive signals for the respective different frequencies.
For this reason, when performing measurement on the cell (different-frequency cell) having a frequency different from that of the cell (serving cell) in which the user equipment is located, the user equipment needs to resynchronize the frequency with that of the cell.
To be more specific, for example, the base station device uses an “RRC message,” which controls measurement, to notify the user equipment of a “length of a gap period,” a “cycle of gap periods,” a “frequency of the different-frequency cell” and the like. In response, the user equipment performs different-frequency measurement (including processes of changing the frequency, acquiring a synchronization channel, measuring the communication quality, changing the frequency, and the like) in the designated gap period.
The “different-frequency measurement” in the present application is a concept covering not only searching for a different-frequency cell and measuring communication quality thereof, but also searching for a cell using a different RAT and measuring communication quality thereof.
For example, in FIG. 2, the base station device may specify, to the user equipment, the second frequency f2 layer and the third frequency f3 layer as the layers to be measured.
As described above, the user equipment performs measurement of the adjacent cell to perform a handover to a cell having the same frequency, a cell having a different frequency or a cell in a different system.
Such measurement is instructed by the network, more specifically, by the base station device. In other words, the user equipment performs measurement of the adjacent cell according to a “measurement configuration” provided by the base station device, and then reports to the base station device a result of measurement of communication quality in the adjacent cell.
Here, the “measurement configuration” is provided to the user equipment in an RRC_Connected state by individual signaling, e.g., an “RRC CONNECTION RECONFIGURATION MESSAGE.”
As described above, the base station device can specify, to the user equipment, multiple layers as the layers to be measured. For example, referring to FIG. 2, the base station device can specify, to the user equipment, the second frequency f2 layer and the third frequency f3 layer as the layers to be measured.
In this case, the user equipment performs measurement of the second frequency f2 layer and measurement of the third frequency f3 layer in the gap period described above. As a measurement method for the layers, there are two methods as shown in FIG. 3: a method for serially performing measurement of the second frequency f2 layer and measurement of the third frequency f3 layer, and a method for performing such measurements in parallel.
When different-frequency measurements of the two layers are serially performed, the time required for the different-frequency measurements is twice as long as the time required for different-frequency measurement on one layer.
On the other hand, when different-frequency measurements of the two layers are performed in parallel, the measurements are performed slowly in terms of time. Accordingly, acquisition of the synchronization channel and measurement of the communication quality take longer compared with the case where the different-frequency measurement is performed on one layer. As a result, the time required for the different-frequency measurements is more than twice as long as the time required for different-frequency measurement on one layer.
In other words, in terms of the time required for the different-frequency measurements, there is an advantage that the time required for the different-frequency measurements can be shortened when the different frequency measurements of the two layers are serially performed as compared with when the different-frequency measurements of the two layers are performed in parallel.
Meanwhile, when the different-frequency measurements of the two layers are serially performed, the different-frequency measurement of the layer with a higher priority or the different-frequency measurement of the layer with a better communication quality is likely to be put off.
For example, suppose that the priority of the third frequency f3 layer is higher than the priority of the second frequency f2 layer in FIG. 3. In this case, the following event occurs. Specifically, when measurement of the second frequency f2 layer is performed before the third frequency f3 layer regardless of the priority, a handover to the second frequency f2 layer is first performed. Then, the measurement of the third frequency f3 layer is performed, and a handover to the third frequency f3 layer is performed.